Generally, a semiconductor device such as a DRAM is integrated over a silicon wafer. However, in the silicon wafer used in the semiconductor device, the whole silicon is not used in the operation of the device, rather a limited thickness of the device, e.g. several μm from the surface, is used. As a result, the rest of the silicon wafer, except a portion required in the operation of the device, increases power consumption and degrades driving speed.
A Silicon On Insulator (SOI) wafer has been required which includes an insulating layer in a silicon substrate to obtain a silicon single crystal layer having thickness of several μm. In a semiconductor device integrated in the SOI wafer, a smaller junction capacity facilitates high speed operation in comparison with a semiconductor device integrated in a general silicon wafer, and a low voltage due to a low threshold satisfies high speed operation and low voltage.